It's rather shocking how many parents don't lock up their (legally owned) guns when kids can get them. On the other hand, when I was a boy, my father had a 22 rifle, usually in the basement (unloaded, but there was ammo somewhere), and I never gave it any thought. I don't know what became of it.
Update: Feb 19:
A subsequent Frontline episode demonstrated the plastic rifle, which can be manufactured in an unregulated fashion with 3-D printers, and will be available soon. This would confound regulatory laws.
There was also discussion of the ease with which criminals get guns in Chicago, despite the strictest gun control laws in the country. 75% of the victims of gun violence in Chicago are black. When a youth has been shot once, the expected time until being shot again (usually in gang activity) is nine months.